The risk of incursions comes from all sides. Rats and mice stow away on recreational boats or in camping gear brought ashore by ferry commuters. And on rare occasions, stoats can swim from the mainland or travel huge distances on driftwood. They have an uncanny ability to make it to land if they fall from a boat.
Even tiny creatures threaten the pest-free idyll of Motuihe. An invasion of Argentine ants at the nearby bird haven of Tiritiri Matangi cost $100,000 to clean up.
Mr Mills said if a rat was transported to the island by ferry, he had 60 metres of breathing space - the length of the jetty - before it was lost into the bush. The ex-navy member would not take any chances: "I'd get someone to watch it and I'd go get my bloody rifle."
The island has seen only one rat in the past five years. Rodents are detected through a series of metre-long plastic tunnels which contain a dob of peanut butter on top of a strip of ink. When the bait is taken, the rat or mouse leaves behind a trail of footprints.
If a rodent is detected, a dog is brought over to the island and let loose next to the tunnel showing rodent prints. In the most recent incursion, two years ago, the specially trained dog found the rat within three hours.
A quick response is crucial, because rats are vicious predators.
"People don't appreciate what they can do," says Motuihe Island Trust volunteer Stuart Macintosh.
"They eat a nest of eggs, or they go up trees and knock a bird off a branch at night - that's very aggressive."
Highly tuned pest control means rat incursions are rare on the island. The greatest threat instead comes from an innocuous source - the families who picnic on the island's white-sand beaches.
John Laurence, chairman of the Motuihe trust, says it is more difficult to monitor the dogs that people bring ashore, or the illegal fires they light.
"A dog could get through and decimate our population of 40 kiwi if left alone.
"Fire is also a significant risk. In a light wind it could go from one end of the island to the other in half an hour."
Despite warnings on VHF radio and clear signs, launch-owners often brought their dogs ashore on Motuihe, Mr Mills said. The animals could pick off a rare bird in minutes.
"If a dog gets a sniff of a kiwi, it's like a man getting a whiff of Chanel No 5. They just exude something that drives dogs crazy."
He used to give visitors with dogs a couple of warnings, but now he spot-fines them $400.
Repeat offenders get their dog impounded.
On the day the Herald visited, the rewards of a pest-free island were immediately evident.
A friendly quail danced cheekily along the walking track, a couple of saddleback sang their high-pitched to-and-fro song, a wood pigeon whirred above us, and a tui sang with its reedy cluck in the top of a lofty pohutukawa.
Native trees were beginning to swallow up the open grassland. All that was missing was bellbird song, which I was assured could be heard when the wind died down.
A volunteer told me she loved Motuihe because "it feels like an island - you can see the sea from wherever you are".
If a 30-knot wind is battering the island from one side, a visitor needs to walk only 50m to reach a more sheltered beach.
And what price this paradise? "It's a bit cheap for a campsite," said Mr Mills, citing the cost of pest control.
"I think we should raise it from $5 to $6."
HISTORY IN BRIEF
Old pa sites found on Motuihe show Maori lived on the island as early as the 14th century, and gravestones at the northwest point are a poignant reminder that it was later a quarantine centre for influenza and scarlet fever.
The island was turned into a prisoner-of-war camp during World War I, when it housed Felix von Luckner, a German war hero, who escaped by stealing the PoW supervisor's speedboat. After the war, it became a naval training base.
PARADISE ISLANDS
Yesterday: Kawau.
Today: Motuihe.
Tomorrow: Motutapu.
Thursday: Little Barrier and Burgess.
Friday: Motiti.
